Natural plant products have served as tremendously valuable tools for deciphering cellular and molecular mechanisms contributing to somatosensation, nociception, and pain. Notable examples include the use of natural analgesics, such as morphine (from the opium poppy) and salicylate (from willow bark) to discover opioid receptors and cyclooxgenases, respectively. Other important examples include the use of natural irritants, such as capsaicin (from chili peppers) and menthol (from mint leaves) to identify ion channels that detect heat and cold, respectively. Indeed, each of these proteins represents a validated or potential target for pharmacological management of acute or chronic pain. Plants are not unique in their capacity to produce chemical agents that target sensory neurons or other excitable cells. Indeed, venoms from animals (ranging from arachnids to mammals) represent a vast pharmacopoeia that has great potential to yield novel agents with which to identify or characterize receptors, ion channels, or other signaling molecules that contribute to sensory transduction. Indeed, in the previous funding period we identified two such toxins - one from spider and another from snake - that serve as novel, potent, and highly selective agonists for TRPV1 and ASIC1 channels, respectively. In each case, these toxins enabled elucidation of the activated, fully open state of the channel at atomic resolution, providing unprecedented insights into structural mechanisms underlying channel gating and modulation. This proposal builds on our success and expertise in toxin discovery and characterization, with the goal of expanding the repertoire of pharmacological agents with which to study known or novel somatosensory receptors. The first aim is focused on characterizing two spider toxins that we identified in a sensory neuron- based screening assay, and which target a specific voltage-gated sodium channel (Nav) subtype expressed by these cells. We propose to identify the Nav channel domain(s) that specifies toxin sensitivity and accounts for its subtype selectivity. Furthermore, we shall test toxin selectivity in vivo using mouse genetics, and identify the subpopulation of sensory neurons that mediate the excitatory and algogenic actions of these toxins in cellular and behavioral paradigms. The second aim is focused on characterizing two novel toxins - one from centipede and the other from snake - that we also identified by functional screening, and which activate primary afferent sensory neurons to elicit nocifensive responses in mice. We propose to identify the molecular targets of these toxins and determine the signaling mechanisms through which they activate sensory neurons of the pain pathway. These studies will uncover novel sensory transduction molecules and/or provide powerful new tools for determining how known transducers work to modulate nociceptor excitability. Information gleaned from this work will provide important pharmacologic leads and insights pertinent to the development of analgesic agents.